Acoustic surface waves measurements by self mixing array transducers for nondestructive testing and structure health monitoring applications
Résumé
In the Non Destructive Testing (NDT) and Structure Health Monitoring (SHM) field, Surface Acoustic Waves (SAWs) are widely
employed to detect defects and/ or to monitor the integrity of the structure to test. In this context, SAWs contactless measurements offer
several advantages with respect to the traditional use of a recording piezoelectric transducers fixed in the structure to test. They can be
easily employed in situations of difficult access to the structure avoiding the generation of undesired reflections due to the presence of a
second transducers mass. The gold standard technique for this this kind of measurements is the scanning Laser Doppler Vibrometer
(sLDV), which allows to perform contactless measurements with a high spatial resolutions and a good Signal to Noise ratio. The main
drawback is the high costs to be supported and the large acquisition time that a 2D grid of measurement points could require. Moreover,
to ensure an optimal reflection of the incident light, the surface of the structure under test is often treated with reflective paint or stickers.
The purpose of this article is to build a large bandwidth array transducer for NDT/SHM applications overcoming the sLDV measurements disadvantages. This technique is based on the use of Laser diodes. The principle behind this approach is the Self Mixing effect
occurring when the optical beam is back-scattered in the active cavity of the diode itself.